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Reductive dechlorination of perchloroethylene in an aerobic sequencing batch reactor

page 87

10 REDUCTIVE DECHLORINATION OF
PERCHLOROETHYLENE IN AN AEROBIC
SEQUENCING BATCH REACTOR
Enos C. Inniss, Doctoral Student
Lloyd Ketchum Jr., Associate Professor
Department of Civil Engineering and Geological Sciences
University of Notre Dame, Notre Dame, Indiana 46556
ABSTRACT
Microorganisms mediate the sequential dechlorination of perchloroethylene (PCE) to
trichloroethylene, dichloroethylene, vinyl chloride, and, ultimately, ethylene, a nontoxic compound. Researchers have focused on PCE dechlorination in strict anaerobic environments while
little or no data have been published on the effects of oxygen concentration on this process.
In this experiment, sequencing batch reactors contained aerobic/facultative anaerobic mixed
cultures which could dechlorinate PCE during anaerobic periods, with glucose as a carbon and
energy source, despite experiencing an aerobic period during the cycle. The acclimated enrichment cultures were able to dechlorinate 90 to 95% of 10 mg/L of PCE to cis-l,2-dichloroethyl-
ene (cDCE). The cultures were exposed to 8-9 mg/L of oxygen for 1.0 hour every 24 hours. This
report represents a major step toward the creation of a single biological reactor than can promote
both PCE dechlorination and oxidation of the dechlorination products.
INTRODUCTION
Tetrachloroethylene (i.e., perchloroethylene or PCE), and its dechlorination products,
trichloroethylene (TCE), dichloroethylene (1,1-, cis-1,2-, and trans-1,2-DCE), and vinyl chloride
(VC), are common groundwater contaminants regulated under the Safe Drinking Water Act
Amendments of 1986.' DiStephano et al., Vogel and McCarty, and others have presented evidence of transformation of PCE, a four chlorine substituted ethylene, to its dechlorination products under strict anaerobic or reducing conditions.24 But PCE does not appear to be transformable under aerobic conditions. Comparatively, its transformation products, namely TCE and
DCE, are more labile to anoxic and aerobic transformation pathways due to the lesser chlorine
substitutions.5"8
This chapter addresses the question of whether a process can be established which is efficient
at utilizing both anaerobic and aerobic processes; i.e., can the two processes occur within the
same reactor? If an environment may be engineered which allows for PCE to be the initial preferential electron acceptor while still allowing some members of the population to maintain their
ability to use other electron acceptors such as oxygen, then it may be possible for PCE to be
dechlorinated and for its products to undergo subsequent transformation, possibly through oxidation and mineralization.
Previous studies at the University of Notre Dame have shown success at reductively dechlori-
nating PCE to cis-DCE as the predominant end product under anaerobic conditions within a 24-
hour period. The results of these experiments have been consistent and reproducible.-1 Our study
tests whether consistent reductive dechlorination may also be achieved in a reactor system which
is not strictly anaerobic. The ability to subject the reactor to both aerobic conditions and anaerobic conditions will allow for the transition from an environment favorable for transforming PCE
52nd Purdue Industrial Waste Conference Proceedings, 1997, Ann Arbor Press, Chelsea, Michigan 48118. Printed in
U.S.A.
87

10 REDUCTIVE DECHLORINATION OF
PERCHLOROETHYLENE IN AN AEROBIC
SEQUENCING BATCH REACTOR
Enos C. Inniss, Doctoral Student
Lloyd Ketchum Jr., Associate Professor
Department of Civil Engineering and Geological Sciences
University of Notre Dame, Notre Dame, Indiana 46556
ABSTRACT
Microorganisms mediate the sequential dechlorination of perchloroethylene (PCE) to
trichloroethylene, dichloroethylene, vinyl chloride, and, ultimately, ethylene, a nontoxic compound. Researchers have focused on PCE dechlorination in strict anaerobic environments while
little or no data have been published on the effects of oxygen concentration on this process.
In this experiment, sequencing batch reactors contained aerobic/facultative anaerobic mixed
cultures which could dechlorinate PCE during anaerobic periods, with glucose as a carbon and
energy source, despite experiencing an aerobic period during the cycle. The acclimated enrichment cultures were able to dechlorinate 90 to 95% of 10 mg/L of PCE to cis-l,2-dichloroethyl-
ene (cDCE). The cultures were exposed to 8-9 mg/L of oxygen for 1.0 hour every 24 hours. This
report represents a major step toward the creation of a single biological reactor than can promote
both PCE dechlorination and oxidation of the dechlorination products.
INTRODUCTION
Tetrachloroethylene (i.e., perchloroethylene or PCE), and its dechlorination products,
trichloroethylene (TCE), dichloroethylene (1,1-, cis-1,2-, and trans-1,2-DCE), and vinyl chloride
(VC), are common groundwater contaminants regulated under the Safe Drinking Water Act
Amendments of 1986.' DiStephano et al., Vogel and McCarty, and others have presented evidence of transformation of PCE, a four chlorine substituted ethylene, to its dechlorination products under strict anaerobic or reducing conditions.24 But PCE does not appear to be transformable under aerobic conditions. Comparatively, its transformation products, namely TCE and
DCE, are more labile to anoxic and aerobic transformation pathways due to the lesser chlorine
substitutions.5"8
This chapter addresses the question of whether a process can be established which is efficient
at utilizing both anaerobic and aerobic processes; i.e., can the two processes occur within the
same reactor? If an environment may be engineered which allows for PCE to be the initial preferential electron acceptor while still allowing some members of the population to maintain their
ability to use other electron acceptors such as oxygen, then it may be possible for PCE to be
dechlorinated and for its products to undergo subsequent transformation, possibly through oxidation and mineralization.
Previous studies at the University of Notre Dame have shown success at reductively dechlori-
nating PCE to cis-DCE as the predominant end product under anaerobic conditions within a 24-
hour period. The results of these experiments have been consistent and reproducible.-1 Our study
tests whether consistent reductive dechlorination may also be achieved in a reactor system which
is not strictly anaerobic. The ability to subject the reactor to both aerobic conditions and anaerobic conditions will allow for the transition from an environment favorable for transforming PCE
52nd Purdue Industrial Waste Conference Proceedings, 1997, Ann Arbor Press, Chelsea, Michigan 48118. Printed in
U.S.A.
87